Evaporative Cooling: Tree Sweat, Bottle Sweat

Using infrared thermometers students investigate the cooling effects of evaporating water. To find examples, we don’t have to look further than, well, sweat. Sweat is critical to keeping our bodies cool when we exercise or spend any time in the heat. When the water in sweat evaporates from our skin it carries off heat. The same principle -- called evaporative cooling -- can be used to cool other things: water, food, even buildings. Some evaporative cooling technologies date back thousands of years, long before electric refrigerators or air conditioners were an option. Evaporative cooling also explains one of the ways trees and other plants can help cool the area around them.

Evaporative Cooling: Tree Sweat, Bottle Sweat was developed as part of the Science of Sweat Workshop, including investigations of how sweat, and more generally evaporating water, can cool things down, and the impacts that extreme heat can have on our bodies. Designed for out-of-school-time educators, the workshop is easily adaptable to other educational settings. Topics include evaporative cooling, the water cycle (including the role of energy), urban heat islands, and staying cool while outdoors. These topics are important to spending time outdoors in the heat, they bring up environmental justice and equity issues, and they provide a hands-on introduction to widely taught science concepts.

• 1 or 2 zip-close plastic bag (quart-size, or any size available)
• 2 metal water bottles
• Unglazed clay flower pot (1 or 2 inches larger than the diameter of the water bottle)
• Cork or stopper (to fit the drainage hole of the flower pot, or tape over the hole with duct tape)
• Clean sand (approximately 3 to 4 cups)
• Optional: unglazed clay water bottle or pitcher (see resources section)
• Student Thermometers, preferably IR Thermometers

Note: IR (infrared) thermometers -- sometimes listed as non-contact thermometers, or “temperature guns” -- allow students to digitally measure the temperature of objects without touching the object. They are wonderful tools for science investigation and can be purchased for $10-$12 and up. Having several will allow students to work in small groups. Choose an IR thermometer that is designed for uses other than (or in addition to) checking body temperature. For example, an IR thermometer that is listed as useful for cooking, refrigeration, or swimming pools. Many models have a built-in laser pointer -- this is helpful for aiming the thermometer, and most models allow you to turn this feature off. You can find models without the laser feature if you are not confident that your students can use a laser pointer safely.

You sweat. Trees sweat… sort of. For plants, it is called transpiring, at least in polite company.

Every leaf has tiny holes on the bottom that can open and close (called stomata). The holes let air in, so the plant can absorb gasses it needs from the air, and also lets the leaf release excess gases. Our lungs do the same thing for us.

When the stomata are open, water also evaporates from inside the leaf. Evaporating water carries heat away from the plant — cooling the plant. In fact, if there are enough plants, they can cool the area around them. What are other ways trees can keep you cool?

You can catch some tree sweat by putting a zip lock bag around a few leaves on a tree. It may take an hour or two (less on a sunny day) but you should start to see some condensing water on the inside of the bag. Lucky for you, tree sweat doesn't smell!

Use the IR thermometers to check the temperature of leaves, and compare them to other surfaces and materials.

Be Safe: Many IR thermometers have a built-in laser pointer to help you aim the thermometer. Never point the laser into your eye, or into the eye of another person or animal.

In many parts of the world, clay water jars are used to keep drinking water cool without a refrigerator. Unglazed clay lets a very small amount of water seep through (it is slightly porous). The water on the outside of the jar then absorbs heat from the jar as it evaporates, removing heat from the jar and cooling the water inside.

This phenomenon was used starting thousands of years ago in areas including India, parts of Africa, and the Near East. There are even ancient Egyptian drawings of people using fans to cool clay water jars.

You can buy a clay water jar or pitcher (see Resources), or follow the directions below to build a “Zeer”. If you have a clay water jar or pitcher, fill it with water and set it in the shade next to a full plastic or metal water bottle. Ideally, set them up an hour or more before students arrive, since it will take some time for the water in the regular water bottle to reach ambient temperature, and the water in the clay water jar to cool down.

Have students use a thermometer to compare the temperature of the water in a clay water jar versus the plastic bottle.

The principle of evaporation cooling can also be used for keeping food cool without electricity. The “clay pot cooler” or “Zeer” is another very old technology for cooling food. You can build your own with an unglazed clay flower pot.

First, plug the drainage hole of the flower pot with a stopper, or tape over it with duct tape. Then fill up a metal water bottle, and stand it in the center of the pot. Fill the gap between the bottle and pot with sand. Finally, pour water on the sand until it is evenly moist.

Like a clay water bottle, the clay flowerpot is porous, allowing some water to seep through to the outside and evaporate. This will slowly cool the pot, sand, and finally the inside container.

Put the Zeer and a regular water bottle side by side in a shady, breezy place. Monitor the temperature of the water in each and see what happens over several hours. Under good conditions, the water in the Zeer will be several degrees cooler -- not as cold as your refrigerator, but still cool!

As with all evaporative cooling, a breeze will help the process. Also, relative humidity is important -- the more humid the surrounding air, the less evaporative cooling will take place. That is why a humid day will feel hotter than a dry day -- even though the temperature may be the same. On a humid day, sweat will not evaporate as quickly, and consequently, our sweating does not cool us as effectively.

(Chemistry note: Why is this true? At the surface of water, some water molecules are always evaporating while others are condensing. Evaporating molecules take in energy/heat, condensing molecules give off energy/heat. The key to cooling is the net evaporation. If the surrounding air is dry, many water molecules in our sweat will evaporate, and relatively few water molecules from the air will condense, so the net evaporation is high. A breeze can also help move humid air away and bring dryer air in, again increasing the net evaporation. On the other hand, if the sounding air has a very high relative humidity, nearly as many water molecules will condense as evaporate -- there will be little net evaporation, and so, little cooling. For more on why evaporation leads to cooling see Water Cycle Simulation, With Energy [link])

About Tree Sweat (Transpiration):

Sweating Can Be Cool | Science Mission Directorate

Use of clay pot evaporation cooling in architecture:

Indian architect looks to nature to cool without chemicals

Evaporative Cooling (clay water jars and Zeer pot coolers):

• You can buy unglazed clay water bottles and pitchers. Search online for “Unglazed clay water pitcher” or “unglazed clay water bottle”; or an unglazed terracotta water pitcher, carafe, or bottle. Make sure the product is unglazed and intended for cooling water. Keep in mind these containers work because they are slightly porous, so a small amount of water will collect under the pitcher.

• This Instructable has a lot of good background information on the concept, but features a more sophisticated design than is necessary for a demonstration:

https://www.instructables.com/A-Practical-Zeer-Pot...

• For more background information on clay pot coolers:

https://www.engineeringforchange.org/news/clay-pot...

• And for even more history:

https://en.wikipedia.org/wiki/Pot-in-pot_refrigera...

This work is made possible by support from STAR, a Biogen Foundation Initiative. The team at Lesley supporting this initiative includes faculty and staff in the Lesley STEAM Learning Lab, Science in Education, the Center for Mathematics Achievement, and other related Lesley University departments and programs.